The invention relates to the use of radiation-curable compositions comprising from 5 to 100% by weight, based on the overall amount of radiation-curable compounds, of carbonate (meth)acrylates as coating compositions for exterior applications.
Radiation-curable compositions have to date been used predominantly for the coating of wood, paper and plastics in the interior sector. Exterior applications require resistance to the effects of weathering, which for example means low yellowing tendency, stability to hydrolysis, mechanical stability under weathering conditions, especially stability to cracking, and also acid and other chemical resistance.
In addition to the required weathering stability there is a desire for good processing properties, such as minimal odor nuisance and high reactivity, and also good mechanical properties in the resultant coatings, such as high flexibility and hardness.
Radiation-curable compositions known to date, as disclosed for example by JP 62-132570 and WO 92/17337, do not go far enough toward meeting the above requirements.
EP-A-92 269 and DE-A-27 57 086 disclose carbonate (meth)acrylates. Exterior applications are not described.
It is an object of the present invention to provide weathering-resistant, radiation-curable compositions for exterior applications, having high weathering stability, good processing properties and, at the same time, good mechanical properties.
We have found that this object is achieved by the use of the above compositions.
The radiation-curable compositions contain from 0.5 to 100% by weight, preferably from 5 to 80% by weight and, with particular preference, from 10 to 50% by weight of carbonate (meth)acrylates. These percentages by weight are based on the sum of radiation-curable compounds.
The carbonate (meth)acrylates contain on average preferably from 1 to 5, in particular from 1 to 3, and, with particular preference, 2 or 3 carbonate groups.
The carbonate (meth)acrylates contain on average preferably from 1 to 5, in particular from 2 to 4 and, with particular preference, 2 or 3 (meth)acrylic groups, and with very particular preference 2 (meth)acrylic groups.
The number-average molecular weight Mn of the carbonate (meth)acrylates is preferably less than 3000 g/mol, with particular preference less than 1500 g/mol and, with very particular preference, less than 800 g/mol (as determined by gel permeation chromatography using polystyrene as standard and tetrahydrofuran as solvent).
The carbonate (meth)acrylates are readily obtainable by transesterifying carbonic esters with polyhydric, preferably dihydric, alcohols (diols, such as hexanediol) and subsequently esterifying the free OH groups with (meth)acrylic acid or else by a transesterification with (meth)acrylic esters, as is described, for example, in EP 92 269. They can also be obtained by reacting phosgene, urea derivatives with polyhydric alcohols, such as dihydric alcohols, for example.
Particularly preferred carbonate (meth)acrylates are those of the formula: 
where R1 is H or CH3, X is a C2-C18 alkylene group and n is an integer from 1 to 5, preferably from 1 to 3.
R 1 is preferably H, and X is preferably C2-C10 alkylene, with particular preference C4-C8 alkylene. With very particular preference, X is C6 alkylene.
These compounds are preferably aliphatic carbonate (meth)acrylates.
In addition to the carbonate (meth)acrylates the radiation-curable composition may include further radiation-curable compounds.
Particularly suitable are urethane (meth)acrylates, epoxy (meth)acrylates and melamine (meth)acrylates.
Urethane (meth)acrylates, for example, are obtainable by reacting polyisocyanates with hydroxyalkyl (meth)acrylates with or without chain extenders such as diols, polyols, diamines, polyamines or dithiols or polythiols. Urethane acrylates which can be dispersed in water without the addition of emulsifiers additionally comprise ionic and/or nonionic hydrophilic groups, which are introduced into the urethane acrylate by means, for example, of structural components such as hydroxycarboxylic acids.
The urethane (meth)acrylates preferably have a number-average molecular weight Mn of from 500 to 20,000, in particular from 750 to 10,000 and, with particular preference, from 750 to 3000 g/mol (as determined by gel permeation chromatography using polystyrene as standard).
The urethane (meth)acrylates preferably have a content of from 1 to 5, with particular preference from 2 to 4, mols of (meth)acrylic groups per 1000 g of urethane (meth)acrylate.
Preferred structural components of the urethane (meth)acrylates are
C1-C8-hydroxyalkyl (meth)acrylates,
C2-C8-alkanediols or -polyols or C2-C8-alkylamines as chain extenders; the urethane acrylates can also include polyesterols or polyetherols as structural components; preferred polyisocyanates are
dicyclohexylmethane 4,4xe2x80x2-diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, tetramethylene diisocyanate, trimethylhexamethylene diisocyanate, adducts of such isocyanates with polyhydric alcohols such as trimethylolpropane, dimers or trimers of the isocyanates, such as biurets or isocyanurates.
Preference is given to aliphatic polyisocyanates, the term aliphatic being intended to include nonaromatic alicyclic compounds as well.
Preferred urethane (meth)acrylates are aliphatic urethane acrylates, which contain aromatic ring systems if at all, in minor amounts of, for example, less than % by weight, based on the urethane acrylates, and which with particular preference contain no aromatic ring systems.
Epoxide (meth)acrylates are obtainable by reacting epoxides with (meth)acrylic acid. Examples of suitable epoxides are epoxidized olefins or glycidyl ethers, examples being bisphenol A diglycidyl ether, or aliphatic glycidyl ethers, such as butanediol diglycidyl ether.
Melamine (meth)acrylates are obtainable by reacting melamine with (meth)acrylic acid.
The epoxy (meth)acrylates and melamine (meth)acrylates preferably have a number-average molecular weight Mn of from 500 to 20,000, with particular preference from 750 to 10,000, and, with very particular preference, from 750 to 3000 g/mol; the content of meth(acrylic) groups is preferably from 1 to 5, with particular preference from 2 to 4, mol per 1000 g of epoxy (meth)acrylate or melamine (meth)acrylate (gel permeation chromatography, PS standard, THF solvent).
Preferred radiation-curable compositions comprise from to 80% by weight, with particular preference from to 50% by weight, of carbonate (meth)acrylates a) and from 20 to 95% by weight, with particular preference from 50 to 90% by weight, of urethane (meth)acrylates, epoxy (meth)acrylates or melamine (meth)acrylates b).
In addition to the above (meth)acrylic compounds the radiation-curable composition may include other ethylenically unsaturated, free-radically polymerizable compounds.
Mention may be made in particular of compounds c), which have at least one (meth)acrylic group but are different from the above (meth)acrylic compounds a) and b).
Suitable compounds c) are poly(meth)acrylates of aliphatic polyols, especially di- to pentahydric alcohols, which other than the hydroxyl groups contain no further functional groups or, at best, ether groups.
Examples of such alcohols are bifunctional alcohols, such as ethylene glycol, and propylene glycol, and the representatives with higher degrees of condensation, such as diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, etc., butanediol, pentanediol, hexanediol, neopentyl glycol, cyclohexanedimethanol, trifunctional alcohols and alcohols of higher functionality, such as glycerol, trimethylolpropane, butanetriol, trimethylolethane, pentaerythritol, ditrimethylolpropane, dipentaerythritol, sorbitol, mannitol and the corresponding alkoxylated alcohols, especially ethoxylated and propoxylated alcohols.
Suitable poly(meth)acrylates in particular have a molecular weight of less than 1000 g/mol, with particular preference of less than 600 g/mol, and have from 2 to 4 (meth)acrylic groups in the molecule; in other words, they are di-, tri- or tetraacrylates.
Further compounds c) having at least one (meth)acrylic group and being different from a) and b) are, for example, polyester (meth)acrylates, or else mono(meth)acrylates, such as butyl acrylate, cyclohexyl acrylate, etc. Monoacrylates are employed in particular as additional reactive diluents.
With all of the above compounds, the acrylic compounds are fundamentally preferred over the methacrylic compounds.
The radiation-curable compositions may also comprise radiation-curable compounds d) without acrylic groups. Suitable examples are vinyl- or alkyl-containing compounds, such as vinyl esters, aromatic vinyl compounds, such as styrene, vinyl ethers, their oligomers or polymers, and also unsaturated polyesters, etc.
Preferably, the overall amount of the radiation-curable compounds is composed as follows:
from to 80, with particular preference from 10 to 50% by weight of a),
from 20 to 95, with particular preference from 50 to 90% by weight of b),
from 0 to 50, with particular preference from 0 to % by weight of c),
from 0 to 50, with particular preference from 0 to 10% by weight of d).
Preferably, the radiation-curable compositions contain little if any aromatic components. The content of aromatic carbons (that is, carbons which are part of an aromatic ring system) is preferably below 5% by weight, with particular preference below 2% by weight, and, with very particular preference, below 0.5% by weight based on the overall amount of the radiation-curable compounds. In particular it is 0%.
The radiation-curable compounds can be present in the radiation-curable compositions in solvent-free form, dissolved in organic solvents, or dispersed in water.
Preference is given to solvent-free systems and especially to aqueous systems.
To this end, the radiation-curable compounds can be dispersed in water with the aid of emulsifiers or protective colloids, where the compounds are not already self-dispersible by virtue of containing hydrophilic groups.
The radiation-curable compositions preferably include at least one photoinitiator for the photochemical polymerization of free-radical polymerizable compounds.
Suitable examples are mono- or bisacylphosphine oxides, benzophenones, hydroxyacetophenones, phenylglyoxylic acid and its derivatives, or mixtures of these photoinitiators.
The radiation-curable compositions contain preferably from 0.1 to parts by weight of photoinitiators, with particular preference from 1 to 3 parts by weight of photoinitiators, based on 100 parts by weight of radiation-curable compounds.
In addition to the photoinitiators and radiation-curable compounds, the radiation-curable compositions may comprise further constituents. Particular mention may be made of pigments, flow control agents and stabilizers. For applications in the exterior sectorxe2x80x94that is, for coatings which are directly exposed to daylightxe2x80x94the compositions will in particular comprise UV absorbers and free-radical scavengers.
UV absorbers convert UV radiation into thermal energy. Known UV absorbers are hydroxybenzophenones, benzotriazoles, cinnamic esters, and oxalanilides.
Free-radical scavengers bind free radicals which form as intermediates. Important free-radical scavengers are sterically hindered amines, which are known as HALS (Hindered Amine Light Stabilizers).
For exterior applications, the content of UV absorbers and free-radical scavengers is preferably in each case from 0.1 to 5 parts by weight, with particular preference from 0.5 to 3 parts by weight and, with very particular preference, from 2 to 3 parts by weight per 100 parts by weight of the radiation-curable compounds.
In addition, the radiation-curable composition may, as well as radiation-curable compounds, include other compounds which contribute to curing by means of different chemical reactions. Suitable examples are polyisocyanates, which crosslink with hydroxyl groups or amino groups.
The radiation-curable compositions are suitable for use as coating compositions which are employed in the exterior sector and which, therefore, are exposed to daylight.
Examples of such applications are the exterior coating of buildings or parts of buildings, especially facings or roof tiles, coatings on vehicles, especially on motor vehicles such as passenger cars, and trucks (in short, automobiles), rail vehicles, and aircraft.
The radiation-curable compositions are used in particular as a clearcoat (also called topcoat) of motor vehicles. The clearcoat of a motor vehicle is the outermost finish coat, which is exposed to weathering.
In this context, the radiation-curable compositions can be used to coat vehicle bodies or individual parts of vehicles consisting, for example, of metal or plastic. The radiation-curable compositions can also be coated onto films and these films can in turn be bonded adhesively to the vehicle, e.g., to the bodywork or parts of a vehicle.
The substrates to be coated can be metallic substrates, plastics, especially in the form of films and sheets, wood, or ceramic substrates.
The radiation-curable coating compositions can be applied to the coating substrates by known methods. Application techniques such as spraying, rolling and knife coating are particularly appropriate.
Curing can take place by radiation curing with UV light, with the UV fraction of daylight possibly being sufficient under certain circumstances. Preferably, however, commercial UV lamps are used for radiation curing. Curing can also take place by means of electron beams.
The coatings produced with the radiation-curable compositions exhibit high resistance to the effects of weathering, in particular a low yellowing tendency, high stability to hydrolysis, and very good mechanical properties, especially high flexibility and hardness.